Chlorine dioxide (ClO2) has many industrial and municipal uses. When produced and handled properly, ClO2 is an effective and powerful biocide, disinfectant and oxidizer.
ClO2 is also used extensively in the pulp and paper industry as a bleaching agent, but is gaining further support in such areas as disinfections in municipal water treatment. Other end-uses can include as a disinfectant in the food and beverage industries, wastewater treatment, industrial water treatment, cleaning and disinfections of medical wastes, textile bleaching, odor control for the rendering industry, circuit board cleansing in the electronics industry, and uses in the oil and gas industry.
In water treatment applications, ClO2 is primarily used as a disinfectant for surface waters with odor and taste problems. It is an effective biocide at low concentrations and over a wide pH range. ClO2 is desirable because when it reacts with an organism in water, chlorite results, which studies to date have shown does not pose a significant adverse risk to human health. The use of chlorine, on the other hand, can result in the creation of chlorinated organic compounds when treating water. Such chlorinated organic compounds are suspected to increase cancer risk.
Producing ClO2 gas for use in a ClO2 water treatment process is desirable because there is greater assurance of ClO2 purity when in the gas phase. ClO2 is, however, unstable in the gas phase and will readily undergo decomposition into chlorine gas (Cl2), oxygen gas (O2), and heat. The high reactivity of ClO2 generally requires that it be produced and used at the same location. ClO2 is, however, soluble and stable in an aqueous solution.
The production of ClO2 can be accomplished both by electrochemical and reactor-based chemical methods. Electrochemical methods have an advantage of relatively safer operation compared to reactor-based chemical methods. In this regard, electrochemical methods employ only one precursor, namely, a chlorite solution, unlike the multiple precursors that are employed in reactor-based chemical methods. Moreover, in reactor-based chemical methods, the use of concentrated acids and chlorine gas poses a safety concern.
Electrochemical cells are capable of carrying out selective oxidation reaction of chlorite to ClO2 The selective oxidation reaction product is a solution containing ClO2. To further purify the ClO2 gas stream, the gas stream is separated from the solution using a stripper column. In the stripper column, air is passed from the bottom of the column to the top while the ClO2 solution travels from top to the bottom. Pure ClO2 is exchanged from solution to the air. Suction of air is usually accomplished using an eductor, as described in copending and co-owned application Ser. No. 10/902,681, of which the present application is a continuation-in-part.
As described in the '681 application, ClO2 can be prepared a number of ways, generally via a reaction involving either chlorite (ClO2—) or chlorate (ClO3—) solutions. The ClO2 created through such a reaction is often refined to generate ClO2 gas for use in the water treatment process. The ClO2 gas is then educed into the water selected for treatment. Eduction occurs where the ClO2 gas, in combination with air, is mixed with the water selected for treatment.
As further described in the '681 application, for many water treatment systems, the eduction process is effective to introduce ClO2 gas directly into the process water. An operational problem can occur, however, when air is simultaneously introduced into a water system while educing the ClO2 gas. A significant corrosion potential results from oxygen in air being added into the system.
Another problem can occur when introducing ClO2 gas into a pressurized water system. Treating water in pressurized systems can be difficult when using educed ClO2 gas, since high-pressure booster pumps may be needed along with high-performance eductors. This not only increases cost, but also raises maintenance concerns, since high-performance eduction systems can be unreliable as operating pressures near or above 30 to 50 pounds per square inch (psi)(206.8 to 344.7 kilopascal (kPa)).
The foregoing eductor-based method is less effective, however, in systems in which a ClO2 stream is directed against a head pressure. To overcome this deficiency, a vacuum gas transfer pump can be employed instead of the eductor described in the '681 application. The size and capacity of the vacuum gas transfer pump are preferably determined by parameters associated with safe, efficient and reliable operation of the generator. In this regard, it has been determined that, for safe, efficient and reliable operation of the generator, a ClO2 concentration of less than about 10 percent by volume of a stream comprising ClO2 in air, the lower decomposition limit, is preferred. To further increase the safety margin of the generator, a ClO2 concentration of less than about 5 percent by volume of a stream comprising ClO2 in air is more desirable.
As the amount of ClO2 produced by the generator increases, the amount of air required for the effective operation of the stripper column also increases. The production range of the generator therefore determines the size of the vacuum gas transfer pump. As the pump size increase the velocity of the mixed air/ClO2 stream exiting the pump increases. Consequently, the temperature of the gas mixture increases.
It is known that ClO2 is unstable and capable of decomposing, in which ClO2 undergoes an exothermic reaction to form chlorine and oxygen. In fact, and as described in more detail below, an operating temperature greater than about 163° F. (73° C.) can result in potentially hazardous and less efficient operation of the generator. In the present technique, in which the ClO2 solution generator has temperature control capability, the operating temperature can be reduced and maintained below the level at which the exothermic reaction to form chlorine and oxygen causes the ClO2 generation process to become hazardous and less efficient.